Typical of encoders in this area are those described in the following publications:
"Dual Plane Capacitive Coupling Encoder", authored by R. J. Flaherty, M. L. Sendelweck, and J. W. Woods, IBM Technical Disclosure Bulletin, Vol. 15, No. 4, Sept. 1972.
"Electrodynamic Velocity and Position Sensor and Emitter Wheel", authored by H. E. Naylor, III, and R. A. Williams, IBM Technical Disclosure Bulletin, Vol 16, No. 10, March 1974.